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Q&A

208VAC triple-phase to single-phase conversion

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The question:

Is there a way to convert from triple-phase 208VAC to single-phase 208VAC?

Background:

Hi everyone, I am stuck on a project I'm working on. I'd like to convert 208VAC (three phase, NEMA L5-20R) to 230VAC (single phase, NEMA 5-20R) for use with a piece of 230VAC equipment that plugs into the 5-20R.

The wall outlet I am limited to using is the aforementioned 208VAC outlet in wye arrangement. Only two of the three 120V hots are accessible, as well as the grounded neutral. 208V is taken by the difference between the two phase-shifted 120V hots. See the diagram below:

Image alt text

The goal is to get a single phase voltage much like how you get 120VAC single phase at a typical NEMA 5-15R outlet you see in a North American residence/business. But instead of 120V, it needs to be 230VAC for the equipment I want to attach. So like this: Image alt text

Not being too experienced in this field, I got a single-phase T-1-81051 autotransformer from Grainger but realized too late that I couldn't take the two three-phase hots from the wall and use it as a pseudo single-phase. I recognized this after looking more at the wiring diagram for it (Fig. H) and realizing that I would be shorting the connections if I made the attachments as shown. Image alt text Image alt text

Everything else matches what I need for the equipment load, but the input not being single-phase is a problem. My question is if there's a way to convert triple-phase to single-phase, maintaining a neutral/ground connection. I want to convert the power from the wall from 208V triple-phase to 208V single-phase before feeding that single-phase into the transformer I got.

I searched a bit and learned about phase converters, but it seemed like they only go the opposite way around, converting from single-phase to triple-phase only.

Any help is appreciated! I know others have much more experience and might be aware of other options out there. I'd be happy to answer any questions! Thanks in advance.


Edit: This shows the voltages I measured using a digital multimeter at the NEMA L5-20 outlet. The voltages agree with a standard three-phase system: Image alt text

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Why do you say convert triple phase to single phase when this is contradicted by only having two of t... (2 comments)

2 answers

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You need a real transformer that converts 208 V to 230 V, so a ratio of 1:1.11. This can not be an auto-transformer because you need isolation between the primary and secondary. That is because you will ground one side of the output, but neither input can be connected to ground.

Alternatively, you could use a transformer that takes only one of the 120 V inputs and converts that to 230 V. This is not preferred because the input currents will be higher, and they will not be balanced between the two phases available to you. This method is acceptable if the total output power is low. For example, if you just want to run a small appliance that needs 230 V, like something temporarily brought over from Europe, then this can be acceptable. However, if this is the case, then an off the shelf "universal" input 230 V UPS is a better choice.

If the output power will be substantial relative to the power capability of the 2-phase feed you have, you really should do it right and convert the 208 V to 230 V.

In any case, you need to consult a competent electrician. At these voltages and powers, there are safety issues and electrical code issues to consider. This is not a do-it-yourself project for anyone that has to ask here.

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Comments on Olin's answer (1 comment)
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I want to convert the power from the wall from 208V triple-phase to 208V single-phase before feeding that single-phase into the transformer I got._

Try this using a 1:1, 208 volts isolation transformer: -

Image alt text

Or get an isolation transformer that directly delivers 230 V AC on the output from a 208 V AC input.

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